This invention relates to a magnetron, more particularly to the magnetron having permanent magnets which are disposed in the vacuum container.
An inner sealed magnet type magnetron of this class may be seen in FIG. 2 of the papers entitled "A Hitachi New Magnetron with an Inserted Magnetic Circuit", which was prepared and reported by Ichiro Inamura et al at International Microwave Power Institute Symposium of May 26, 1977. In this magnetron, the permanent magnet may be disposed more closely to the interaction space which is defined between the cathode and vanes, so that the leakage of magnetic flux in the magnetic circuit becomes smaller and the magnetic flux generated from permanent magnets may be effectively made use of. Thus, a smaller permanent magnet may comply with requirement for magnetron use, and this contributes to lessening the entire dimension of the magnetron. For this reason, the magnetron disclosed in the above papers is now establishing a good reputation.
It is well known, however, that in response to the change of magnetic flux the inner wall of the outer casing which contains the vacuum space or the parts which are disposed therein allows an electric current to flow through their surface. Accordingly, if there is an imperfect connection between the parts contained in the vacuum space, an electric field would be concentrated on such imperfect connecting portion and it would cause a spark discharge eventually. In the magnetron as mentioned above, such sparking phenomenon is apt to be observed at the portion between the pole piece and the holder which is supported by the inner wall of an anode cylinder constituting a part of the outer casing. This depends on the following. The parts other than the holder and the pole piece may be processed and manufactured with comparactively high accuracy whereas the holder and the pole piece may be not. Generally, these two parts, the holder and pole piece, are manufactured through press work in view of their manufacturing cost. Further, the holder is made by using a metal plate having the thickness of from 0.2 to 0.5 mm in consideration of its strength. Consequently, some warps due to the press work occur at certain portions of the holder, for example at the opening to receive the pole piece projecting therethrough into the interaction space or at the opening for the output antenna. What is worse, such warps are apt to concentrate on the peripheral portion of the opening with which the pole piece is going to contact. Thanks to these warps introduced, it becomes very hard to obtain a contact between the holder and the pole piece in such a preferable mode that both parts entirely and uniformly contact to each other or that both parts contact to each other through contacting points which are uniformly distributed along the circumference of a coaxial circle about the opening. Imperfect contact, especially the contact which is extremely localized due to the warp, is helpful for causing the spark discharge phenomenon. Especially, where small quantity of food is cooked in an electronic oven including the magnetron, a large amount of reflected microwave is returned from the antenna so that the electric field at the above-mentioned imperfect contacting point is extremely enhanced thereby the intensive spark discharge being initiated to melt materials locally at such point eventually. As a consequence, gas is discharged to destroy a heated cathode filament. Where the pole piece is melted, it is deformed so that magnetric field distribution becomes worse thus inviting destruction of the magnetron. Further, when the spark discharge is initiated, the heat generated thereby elevates the temperature of the permanent magnet up to more than 300.degree. C. by the help of thermal conduction. In general, the permanent magnet for magnetron use is made of rare earth elements of samarium.cobalt system. When the permanent material made of such material is heated at the temperature of more than 300.degree. C., magnetism is permanently faded away, thus the magnetron being made inoperative. The above discussion has been proceeded in connection with the output side of the magnetron. However, same discussion is possible as to the magnet and holder in the input side of the magnetron.